[PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Sasha Levin]

Building an i386 allmodconfig kernel with clang fails since allmodconfig
started building netfilter with GCOV profiling:

  In file included from net/netfilter/nf_synproxy_core.c:7:
  In file included from ./include/linux/skbuff.h:26:
  In file included from ./include/net/checksum.h:21:
  In file included from ./arch/x86/include/asm/checksum.h:9:
  ./arch/x86/include/asm/checksum_32.h:149:6: error: inline assembly requires more registers than available

The csum_ipv6_magic() asm needs five registers at once: the early-clobber
sum plus register-only inputs for saddr, daddr, htonl(len) and
htonl(proto). With frame pointers enabled, six general purpose registers
are left to choose from.

In synproxy_send_tcp_ipv6(), the GCOV instrumentation lets clang's
machine scheduler move the copies of two incoming register arguments,
EDX and ECX, below the asm. Both registers then stay live across the
asm, which leaves four registers for its five operands. This is LLVM
issue #206307. The error goes away without either GCOV or frame
pointers, and it reproduces with clang 19, 21 and 22.

Neither len nor proto has to be in a register: adcl also takes a 32-bit
memory or immediate source operand. Use "g" for both, so the asm needs
at most three registers whatever the caller looks like.

ASM_INPUT_G, which normally replaces "g" to avoid clang's poor handling
of it, is not enough here. It expands to "ir" for clang, so callers that
pass a variable protocol, like nf_ip6_checksum(), would still need five
registers. With "g", clang passes a non-constant len or proto through
the stack and still turns constants into immediates.

Fixes: 403cec8ab6d0 ("netfilter: add option for GCOV profiling")
Reported-by: kernelci.org bot <bot@kernelci.org>
Closes: https://dashboard.kernelci.org/build/maestro:6ad55fe255fda507603d2651
Reported-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
Closes: https://lore.kernel.org/a28df759-d0e9-41c0-97c7-5f89a18d54e2@oss.qualcomm.com
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>
Cc: stable@vger.kernel.org
Link: https://github.com/llvm/llvm-project/issues/206307
---
KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
with this error since the 7.2 merge window, v7.3 included.

Tested on i386 with KernelCI's allmodconfig-based .config: with clang
19.1.7, 21.1.8 and 22.1.8, nf_synproxy_core.o fails before this change
and builds after it, and net/{netfilter,ipv4,ipv6,bridge}/ and
arch/x86/lib/ build without errors or warnings. The same directories
also build cleanly with GCC 16.2 (i386 allmodconfig). A 32-bit userspace
harness comparing the asm against a C reference over 2M random and
carry-edge inputs shows no mismatches with either compiler. No full
allmodconfig build or boot test.

 arch/x86/include/asm/checksum_32.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
index 17da953879976..f74873a5e3510 100644
--- a/arch/x86/include/asm/checksum_32.h
+++ b/arch/x86/include/asm/checksum_32.h
@@ -159,7 +159,7 @@ static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
 	    "adcl $0, %0	;\n"
 	    : "=&r" (sum)
 	    : "r" (saddr), "r" (daddr),
-	      "r" (htonl(len)), "r" (htonl(proto)), "0" (sum)
+	      "g" (htonl(len)), "g" (htonl(proto)), "0" (sum)
 	    : "memory");
 
 	return csum_fold(sum);
-- 
2.53.0

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Rosen Penev]

On Mon, Oct 19, 2026 at 5:32 PM Sasha Levin <sashal@kernel.org> wrote:
>
> Building an i386 allmodconfig kernel with clang fails since allmodconfig
> started building netfilter with GCOV profiling:
>
>   In file included from net/netfilter/nf_synproxy_core.c:7:
>   In file included from ./include/linux/skbuff.h:26:
>   In file included from ./include/net/checksum.h:21:
>   In file included from ./arch/x86/include/asm/checksum.h:9:
>   ./arch/x86/include/asm/checksum_32.h:149:6: error: inline assembly requires more registers than available
>
> The csum_ipv6_magic() asm needs five registers at once: the early-clobber
> sum plus register-only inputs for saddr, daddr, htonl(len) and
> htonl(proto). With frame pointers enabled, six general purpose registers
> are left to choose from.
>
> In synproxy_send_tcp_ipv6(), the GCOV instrumentation lets clang's
> machine scheduler move the copies of two incoming register arguments,
> EDX and ECX, below the asm. Both registers then stay live across the
> asm, which leaves four registers for its five operands. This is LLVM
> issue #206307. The error goes away without either GCOV or frame
> pointers, and it reproduces with clang 19, 21 and 22.
>
> Neither len nor proto has to be in a register: adcl also takes a 32-bit
> memory or immediate source operand. Use "g" for both, so the asm needs
> at most three registers whatever the caller looks like.
>
> ASM_INPUT_G, which normally replaces "g" to avoid clang's poor handling
> of it, is not enough here. It expands to "ir" for clang, so callers that
> pass a variable protocol, like nf_ip6_checksum(), would still need five
> registers. With "g", clang passes a non-constant len or proto through
> the stack and still turns constants into immediates.
>
> Fixes: 403cec8ab6d0 ("netfilter: add option for GCOV profiling")
> Reported-by: kernelci.org bot <bot@kernelci.org>
> Closes: https://dashboard.kernelci.org/build/maestro:6ad55fe255fda507603d2651
> Reported-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
> Closes: https://lore.kernel.org/a28df759-d0e9-41c0-97c7-5f89a18d54e2@oss.qualcomm.com
> Assisted-by: LLM
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> Cc: stable@vger.kernel.org
> Link: https://github.com/llvm/llvm-project/issues/206307
> ---
> KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
> with this error since the 7.2 merge window, v7.3 included.
>
> Tested on i386 with KernelCI's allmodconfig-based .config: with clang
> 19.1.7, 21.1.8 and 22.1.8, nf_synproxy_core.o fails before this change
> and builds after it, and net/{netfilter,ipv4,ipv6,bridge}/ and
> arch/x86/lib/ build without errors or warnings. The same directories
> also build cleanly with GCC 16.2 (i386 allmodconfig). A 32-bit userspace
> harness comparing the asm against a C reference over 2M random and
> carry-edge inputs shows no mismatches with either compiler. No full
> allmodconfig build or boot test.
>
>  arch/x86/include/asm/checksum_32.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
> index 17da953879976..f74873a5e3510 100644
> --- a/arch/x86/include/asm/checksum_32.h
> +++ b/arch/x86/include/asm/checksum_32.h
> @@ -159,7 +159,7 @@ static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>             "adcl $0, %0        ;\n"
>             : "=&r" (sum)
>             : "r" (saddr), "r" (daddr),
> -             "r" (htonl(len)), "r" (htonl(proto)), "0" (sum)
> +             "g" (htonl(len)), "g" (htonl(proto)), "0" (sum)
>             : "memory");
Same as https://lore.kernel.org/all/20260904000833.2058186-1-rosenp@gmail.com/
>
>         return csum_fold(sum);
> --
> 2.53.0
>

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Borislav Petkov]

On Mon, Oct 19, 2026 at 05:56:44PM -0700, Rosen Penev wrote:
> On Mon, Oct 19, 2026 at 5:32 PM Sasha Levin <sashal@kernel.org> wrote:
> >
> > Building an i386 allmodconfig kernel with clang fails since allmodconfig
> > started building netfilter with GCOV profiling:
> >
> >   In file included from net/netfilter/nf_synproxy_core.c:7:
> >   In file included from ./include/linux/skbuff.h:26:
> >   In file included from ./include/net/checksum.h:21:
> >   In file included from ./arch/x86/include/asm/checksum.h:9:
> >   ./arch/x86/include/asm/checksum_32.h:149:6: error: inline assembly requires more registers than available
> >
> > The csum_ipv6_magic() asm needs five registers at once: the early-clobber
> > sum plus register-only inputs for saddr, daddr, htonl(len) and
> > htonl(proto). With frame pointers enabled, six general purpose registers
> > are left to choose from.
> >
> > In synproxy_send_tcp_ipv6(), the GCOV instrumentation lets clang's
> > machine scheduler move the copies of two incoming register arguments,
> > EDX and ECX, below the asm. Both registers then stay live across the
> > asm, which leaves four registers for its five operands. This is LLVM
> > issue #206307. The error goes away without either GCOV or frame
> > pointers, and it reproduces with clang 19, 21 and 22.
> >
> > Neither len nor proto has to be in a register: adcl also takes a 32-bit
> > memory or immediate source operand. Use "g" for both, so the asm needs
> > at most three registers whatever the caller looks like.
> >
> > ASM_INPUT_G, which normally replaces "g" to avoid clang's poor handling
> > of it, is not enough here. It expands to "ir" for clang, so callers that
> > pass a variable protocol, like nf_ip6_checksum(), would still need five
> > registers. With "g", clang passes a non-constant len or proto through
> > the stack and still turns constants into immediates.
> >
> > Fixes: 403cec8ab6d0 ("netfilter: add option for GCOV profiling")
> > Reported-by: kernelci.org bot <bot@kernelci.org>
> > Closes: https://dashboard.kernelci.org/build/maestro:6ad55fe255fda507603d2651
> > Reported-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
> > Closes: https://lore.kernel.org/a28df759-d0e9-41c0-97c7-5f89a18d54e2@oss.qualcomm.com
> > Assisted-by: LLM
> > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > Cc: stable@vger.kernel.org
> > Link: https://github.com/llvm/llvm-project/issues/206307
> > ---
> > KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
> > with this error since the 7.2 merge window, v7.3 included.

This'll tell you how much people including us care about 32-bit...

> > Tested on i386 with KernelCI's allmodconfig-based .config: with clang
> > 19.1.7, 21.1.8 and 22.1.8, nf_synproxy_core.o fails before this change
> > and builds after it, and net/{netfilter,ipv4,ipv6,bridge}/ and
> > arch/x86/lib/ build without errors or warnings. The same directories
> > also build cleanly with GCC 16.2 (i386 allmodconfig). A 32-bit userspace
> > harness comparing the asm against a C reference over 2M random and
> > carry-edge inputs shows no mismatches with either compiler. No full
> > allmodconfig build or boot test.
> >
> >  arch/x86/include/asm/checksum_32.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
> > index 17da953879976..f74873a5e3510 100644
> > --- a/arch/x86/include/asm/checksum_32.h
> > +++ b/arch/x86/include/asm/checksum_32.h
> > @@ -159,7 +159,7 @@ static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
> >             "adcl $0, %0        ;\n"
> >             : "=&r" (sum)
> >             : "r" (saddr), "r" (daddr),
> > -             "r" (htonl(len)), "r" (htonl(proto)), "0" (sum)
> > +             "g" (htonl(len)), "g" (htonl(proto)), "0" (sum)
> >             : "memory");
> Same as https://lore.kernel.org/all/20260904000833.2058186-1-rosenp@gmail.com/

Except that Sasha's commit message explains it perfectly clear how this
happens while for yours I would've asked how you're reproducing this.

OTOH, I'm not sure I care about Sasha's AI's blurb about what params should've
been in what registers and so on... :)

If only you guys would agree on how to merge the two, assign authorship and
drop the unnecessary blurb...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Sasha Levin]

On Mon, Oct 19, 2026 at 06:11:40PM -0700, Borislav Petkov wrote:
>> On Mon, Oct 19, 2026 at 5:32 PM Sasha Levin <sashal@kernel.org> wrote:
>> > KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
>> > with this error since the 7.2 merge window, v7.3 included.
>
>This'll tell you how much people including us care about 32-bit...

I'm not stopping you from dropping it :)

>> Same as https://lore.kernel.org/all/20260904000833.2058186-1-rosenp@gmail.com/
>
>Except that Sasha's commit message explains it perfectly clear how this
>happens while for yours I would've asked how you're reproducing this.
>
>OTOH, I'm not sure I care about Sasha's AI's blurb about what params should've
>been in what registers and so on... :)
>
>If only you guys would agree on how to merge the two, assign authorship and
>drop the unnecessary blurb...

Any objection to just taking Rosen's? Maybe with the tags I had on mine to make
tracking easier?

-- 
Thanks,
Sasha

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Borislav Petkov]

On Mon, Oct 19, 2026 at 09:17:09PM -0400, Sasha Levin wrote:
> I'm not stopping you from dropping it :)

I'd love to.

> Any objection to just taking Rosen's? Maybe with the tags I had on mine to make
> tracking easier?

Thanks, that's nice of you.

I'd like to use a trimmed version of your commit message because it explains
the big picture better as to why we need this in the first place.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Rosen Penev]

On Mon, Oct 19, 2026 at 6:29 PM Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Oct 19, 2026 at 09:17:09PM -0400, Sasha Levin wrote:
> > I'm not stopping you from dropping it :)
>
> I'd love to.
>
> > Any objection to just taking Rosen's? Maybe with the tags I had on mine to make
> > tracking easier?
>
> Thanks, that's nice of you.
>
> I'd like to use a trimmed version of your commit message because it explains
> the big picture better as to why we need this in the first place.
Feel free.
>
> Thx.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

Re: [PATCH] x86/csum: Relax the len and proto constraints in 32-bit csum_ipv6_magic()

[Nick Desaulniers]

On Mon, Oct 19, 2026 at 5:32 PM Sasha Levin <sashal@kernel.org> wrote:
>
> Building an i386 allmodconfig kernel with clang fails since allmodconfig
> started building netfilter with GCOV profiling:
>
>   In file included from net/netfilter/nf_synproxy_core.c:7:
>   In file included from ./include/linux/skbuff.h:26:
>   In file included from ./include/net/checksum.h:21:
>   In file included from ./arch/x86/include/asm/checksum.h:9:
>   ./arch/x86/include/asm/checksum_32.h:149:6: error: inline assembly requires more registers than available
>
> The csum_ipv6_magic() asm needs five registers at once: the early-clobber
> sum plus register-only inputs for saddr, daddr, htonl(len) and
> htonl(proto). With frame pointers enabled, six general purpose registers
> are left to choose from.
>
> In synproxy_send_tcp_ipv6(), the GCOV instrumentation lets clang's
> machine scheduler move the copies of two incoming register arguments,
> EDX and ECX, below the asm. Both registers then stay live across the
> asm, which leaves four registers for its five operands. This is LLVM
> issue #206307. The error goes away without either GCOV or frame
> pointers, and it reproduces with clang 19, 21 and 22.
>
> Neither len nor proto has to be in a register: adcl also takes a 32-bit
> memory or immediate source operand. Use "g" for both, so the asm needs
> at most three registers whatever the caller looks like.
>
> ASM_INPUT_G, which normally replaces "g" to avoid clang's poor handling
> of it, is not enough here. It expands to "ir" for clang, so callers that
> pass a variable protocol, like nf_ip6_checksum(), would still need five
> registers. With "g", clang passes a non-constant len or proto through
> the stack and still turns constants into immediates.
>
> Fixes: 403cec8ab6d0 ("netfilter: add option for GCOV profiling")
> Reported-by: kernelci.org bot <bot@kernelci.org>
> Closes: https://dashboard.kernelci.org/build/maestro:6ad55fe255fda507603d2651
> Reported-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
> Closes: https://lore.kernel.org/a28df759-d0e9-41c0-97c7-5f89a18d54e2@oss.qualcomm.com
> Assisted-by: LLM
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> Cc: stable@vger.kernel.org
> Link: https://github.com/llvm/llvm-project/issues/206307\

Acked-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
> KernelCI's kbuild-clang-21-i386-allmodconfig job has failed on mainline
> with this error since the 7.2 merge window, v7.3 included.
>
> Tested on i386 with KernelCI's allmodconfig-based .config: with clang
> 19.1.7, 21.1.8 and 22.1.8, nf_synproxy_core.o fails before this change
> and builds after it, and net/{netfilter,ipv4,ipv6,bridge}/ and
> arch/x86/lib/ build without errors or warnings. The same directories
> also build cleanly with GCC 16.2 (i386 allmodconfig). A 32-bit userspace
> harness comparing the asm against a C reference over 2M random and
> carry-edge inputs shows no mismatches with either compiler. No full
> allmodconfig build or boot test.
>
>  arch/x86/include/asm/checksum_32.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
> index 17da953879976..f74873a5e3510 100644
> --- a/arch/x86/include/asm/checksum_32.h
> +++ b/arch/x86/include/asm/checksum_32.h
> @@ -159,7 +159,7 @@ static inline __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>             "adcl $0, %0        ;\n"
>             : "=&r" (sum)
>             : "r" (saddr), "r" (daddr),
> -             "r" (htonl(len)), "r" (htonl(proto)), "0" (sum)
> +             "g" (htonl(len)), "g" (htonl(proto)), "0" (sum)
>             : "memory");
>
>         return csum_fold(sum);
> --
> 2.53.0
>


-- 
Thanks,
~Nick Desaulniers